Field of the Invention
The invention lies in the field of semiconductor technology and relates, more specifically, to a semiconductor configuration.
Silicon carbide (SiC) in monocrystalline form is a semiconductor material with outstanding physical properties which make this semiconductor material particularly useful in high temperature electronics and power electronics.
A power MISFET (Metal Insulator Semiconductor Field Effect Transistor) on SiC basis is known in the art. That transistor has a substrate made of monocrystalline SiC of the 4H or 6H polytype and an epitaxial layer made of SiC which is arranged on a substrate surface of the substrate and is of a predefined conductivity type (n-type or p-type conductivity) and is of the same polytype as the SiC substrate. In the SiC epitaxial layer, a base region of the opposite conductivity type to the SiC epitaxial layer is produced by ion implantation, and a source region of the same conductivity type as the SiC epitaxial layer is produced in the base region by ion implantation. The base region and source region are brought into contact by a source electrode and short-circuited to one another. A silicon dioxide layer (oxide layer) as insulator layer of the MIS structure is arranged on a surface of the base region, and a gate electrode is arranged, as metal layer of the MIS structure, on the oxide layer. By applying an electrical field to the gate electrode, a channel region which is near to the surface in the base region underneath the oxide layer can be controlled in terms of its electrical resistance. To be precise, when there is a specific control field, the conductivity type of the channel region changes as a result of charge-carrier inversion and a conductive channel of the same conductivity type as the source region and SiC epitaxial layer (drift region) is produced on the surface of the base region. The MISFET is then in its on state in which an electric current flows through the channel region to the drain electrode when an operating voltage is applied between the source electrode and a drain electrode. The drain electrode can be arranged on the rear of the substrate (vertical MISFET) or on the same surface as the source electrode (lateral MISFET).
U.S. Pat. No. 5,011,549 to Kong et al. discloses a method for the homoepitaxial deposition growth of an SiC epitaxial layer on an Alpha-SiC (.alpha.-SiC) substrate by means of chemical gas phase deposition (CVD=Chemical Vapor Deposition). The term Alpha-SiC or .alpha.-SiC covers all the polytypes with a monocrystalline SiC with a hexagonal or rhombohedral crystalline structure, which are usually used in this context. In that prior art epitaxial method, the substrate surface of the SiC substrate is prepared before the deposition of the SiC epitaxial layer by mechanical processing, for example sawing, in such a way that the surface is tilted at a predefined angle between 3.degree. and 6.degree. toward one of the two {0001} crystal faces (or one of the associated &lt;0001&gt; crystal directions), that is to say toward the (0001) crystal face (Si side) or toward the (0001) crystal face (C side) in the direction of one of the &lt;1120&gt; crystal directions (misorientation, "off-axis orientation"). On the substrate surface which is prepared in this way, the SiC epitaxial layer is deposited from a process gas mixture containing silicon and carbon by means of a CVD process at temperatures between 1400.degree. C. and 1700.degree. C. The SiC epitaxial layers which are manufactured with this misoriented epitaxial method have, given suitable process control, a polytype which is indeed the same as the SiC substrate, and are of better crystallographic quality, and their conductivity can be set more precisely, than comparable layers which are grown directly on one of the {0001} crystal faces themselves. However, owing to the growth mechanism which is controlled in steps, the SiC epitaxial layers which are grown with misorientation have on their surfaces microscopic steps and edges whose width and height depend on the tilt angle of the substrate surface.
In Materials Research Society Symposium Proceedings, Vol. 162, 1990, pages 397-407, an epitaxial growth of 6H-SiC layers on 6H-SiC substrates which are misoriented toward the (0001) crystal faces by an angle between 3.degree. and 6.degree., by means of CVD at a temperature of 1500.degree. C., is described. When the substrate surface is tilted in the direction of the [1120] crystal direction of the 6H-SiC substrate, microsteps with a zigzag-line shape are produced on the surface of the grown 6H-SiC epitaxial layer, the main direction of extent of the microsteps being directed in each case parallel with the [1110] crystal direction of the 6H-SiC epitaxial layer and their individual zigzag sections running parallel with the hexagonal crystal edges. In contrast, when the substrate surface is tilted in a direction of rotation with respect to the [1110] crystal direction of the 6H-SiC substrate, linear microsteps, which are each directed parallel with the [1120] crystal direction of the 6H-SiC epitaxial layer are found on the surface of the 6H-SiC epitaxial layer.
Applied Physics Letters, Vol. 66, No. 26, Jun. 26, 1995, pages 3645-47 describes, in the context of a misoriented epitaxial method, not only the production of microsteps but also the accumulation of microsteps ("step bunching") to form relatively large hill-and-valley macrostep structures on the surface of 4H-SiC and 6H-SiC epitaxial layers. The 4H-SiC and 6H-SiC epitaxial layers are deposited on 4H and/or 6H-SiC substrates, which are misoriented by 3.degree. to 10.degree. with respect to the [1120] crystal direction, by means of CVD at a temperature of 1500.degree. C. The height of the microsteps which are produced during the growth is always an integral multiple of an Si--C double atom layer (approximately 0.25 nm) owing to the growth mechanism, and it is on average three double layers in the case of 6H-SiC and on average four double layers in the case of 4H-SiC. The width of the microsteps is on average 12 nm in the case of 6H-SiC and 17 nm in the case of 4H-SiC. Corrugated or hill-and-valley macrostep structures have been observed in the case of epitaxial growth on a substrate surface which is tilted toward the (0001) Si face, but not in the case of growth on a substrate surface which is tilted toward the [0001] C face. The probability of the macrosteps being formed decreases the greater the tilt angle of the misorientation of the substrate surface. The measured height of the macrostep is 3 nm in the case of 6H-SiC epitaxial layers, and 10 nm to 15 nm in the case of 4H-SiC epitaxial layers. The width of the terraces between the steps was found to be 280 nm in the case of 6H-SiC and between 110 nm and 160 nm in the case of 4H-SiC.